Static dissipative mat

ABSTRACT

The invention contemplates a multi-layer electrically conductive web or mat for safely and quickly discharging personnel-accumulated static electricity. An upper continuous polymeric layer of relatively low conductivity, in the order of 10 8  ohms per square, is bonded to an underlying continuous polymeric layer of much greater conductivity, in the order of 10 2  ohms per square, and provision is made for electrically grounding the underlying layer, illustratively by casting to the intermediate layer a continuous bottom layer of expanded polymeric material of relatively low conductivity.

BACKGROUND OF THE INVENTION

The invention relates to a static-dissipative web construction, suitablefor example as a floor mat to enable personnel-accumulated staticelectricity to safely discharge from a person standing on the mat.

Various static-discharge mat constructions have been proposed, rangingfrom such highly conductive configurations as to permit the hazard ofsubstantially instantaneous discharge, to slow-leaking constructionswhich exhibit undesirable dependence upon ambient humidity. Betweenthese extremes, U.S. Pat. No. 4,208,696 to Lindsay, et al. describes amulti-layer static-dissipative web wherein an open-weave fabric in theform of cotton scrim is rendered electrically conductive (using carbonin a latex binder) and is interposed between upper and lower layers ofrelatively low conductivity, to produce mat constructions having anoverall volume resistivity between 10¹⁰ and 10¹¹ ohm-cm and surfaceresistance in the order of 10⁸ ohms per square; Lindsay et al. predicatetheir results on the foraminous nature of their conductive open-weavefabric. While the Lindsay, et al. product is in many respectssatisfactory, it is prone to delamination, and for many applications anorder of magnitude reduction in surface resistance is desirable, i.e.,to the order of 10⁷ ohms per square.

BRIEF STATEMENT OF THE INVENTION

It is an object of the invention to provide an improvedstatic-dissipative web or mat of the character indicated.

It is a specific object to produce such a web or mat that is inherentlynot prone to delamination and which exhibits a surface resistance in theorder of 10⁷ ohms per square, as measured pursuant to ASTM StandardD257-76.

A general object is to provide such a web or mat construction which isof relative simplicity, which uses readily controllable and availablecomponent materials, and which is inherently relatively insensitive toenvironmental humidity.

The invention achieves the foregoing objects by employing a thincontinuous film of graphite-compounded polymeric material such as abonded conductive flexible plate on the underside of the upper polymericlayer of low conductivity, the thin film having a surface resistance inthe order of 10² ohms per square, as measured pursuant to said ASTMStandard D257-76. By employing polymeric material of the same nature,e.g., polyvinyl chloride in both these layers, as well as in an expandedconductive cushioning bottom layer, laminar bonding is optimized, andsensitivity to varying humidity is minimized. Also, the continuousnature of all layers, and the continuous nature of their interfacebonding, provides a continuous volume within which electrostatic chargemay dissipate and distribute over the thin film of the conductiveintermediate layer, as distinguished from the discrete paths in whichcharge dissipation must be channeled in the foraminous-scrim networkconfiguration of Lindsay, et al.

DETAILED DESCRIPTION

The invention will be described in detail for a preferred embodiment, inconjunction with the accompanying drawings, in which:

FIG. 1 is an enlarged sectional view through a mat construction of theinvention;

FIG. 2 is an exploded view in perspective, to permit betteridentification of components of the construction of FIG. 1;

FIGS. 3a to 3f are views similar to FIG. 1, to illustrate successivestages in fabrication of the construction of FIG. 1; and

FIG. 4 is a schematic diagram to illustrate steps in a continuousprocess for making the construction of FIG. 1.

In FIGS. 1 and 2, the invention is shown in the form of astatic-dissipating web or mat comprising three bonded layers 10-11-12 ofdifferently conductive polymeric material, such as polyvinyl chloride.The top or upper layer 10 and the bottom or lower layer 12 may each beof the same solid-cast construction, but as shown, the lower layer 12 isan expanded version of the same polymeric material; and both layers10-12 incorporate one or more conductive ingredients to enable each oflayers 10-12 to have a volume resistivity in the range 10⁷ to 10¹²ohm-cm. The inner or intermediate layer 11 is a thin film of preferablythe same polymeric material containing an electrically conductiveingredient such as carbon black and exhibiting a surface resistance inthe order of 10² ohms per square, as measured pursuant to ASTM StandardD257-76. A highly satisfactory conductive vinyl film for use atconductive layer 11 is known as Condulon, a trademark and product ofPervel Industries, Inc., Plainfield, Connecticut.

In a specific illustrative example, the solid upper and expanded lowerlayers 10-12 are both of polyvinyl chloride, with added conductiveplasticizer, which may be commercially available products known asMarkstat AL-15 or di-octyl-adipate (DOA)*, or a combination of the two.If mixed, it is preferred that the proportion by weight of the AL-15 tothe DOA be 2:1, their combination accounting for 12 percent of the totaldry-ingredient mix. In the illustrative example, the solid top layer 10was 35 mils thick, the inner layer 11 was 2 mils thick, and the expandedlower layer was 88 mils thick, for an overall thickness of 125 mils. Theupper and lower layers each exhibited a volume resistivity of about 10⁹ohm-cm and a surface resistance of 10⁸ ohms per square, and the surfaceresistivity of the intermediate layer was 300 ohms per square. Overallsurface resistance of the consolidated mat was measured at the exposedsurface of top layer 10 to be 10⁷ ohms per square, and the time fordischarge of a 5 kV potential was 0.05 second, for a decay to tenpercent of the original charge, per Federal Test Method Standard No.101B, Method 4046.

Continuous manufacture of the described web will be described inconnection with FIGS. 3 and 4, commencing with a supply reel 15 ofsuitable casting web 16 continuously advancing from left to right, inthe sense of FIG. 4. The casting web 16 may be a release-coated fabricor paper, and in the latter event the casting surface thereof ispreferably embossed (as in FIG. 3a), for ultimate aesthetic purposes atthe exposed upper surface of layer 10.

A first casting of liquid-mixed polymeric-coat ingredients is made at 17and the same is cured or fused at 18, thereby establishing the solidlayer 10 atop the casting paper 16, as shown in FIG. 3b. A separatesupply of conductive film for inner layer 11 is available from a reel 19and is guided for bonded lamination to layer 10, under heat andpressure, at heated squeeze rolls 20, to produce intermediate productshown in FIG. 3c. A second casting of liquid-mixed polymeric-coatingredients (this time with an expanding component) is then made at 21,so that the developing product appears as in FIG. 3d, wherein thenumeral 12' will be understood to designate the as-yet unexpanded liquidcoat applied at 17. Passage through an expanding oven 22 enablescontrolled uniform expansion of the coat 12' to its ultimate thickness,as bottom layer 12, the same being consolidated in a fusing oven 23,with the appearance shown in FIG. 3e. The product is now completed bystripped removal of the casting paper and its separate accumulation at24, leaving finished product (FIG. 3f) available for reel accumulationat 25.

Static-dissipating vinyl mat material, produced as described is found tomeet all stated objects. Static-dissipating conductivity is an order ofmagnitude (i.e., 10 times) better, that is, a surface resistance of 10⁷ohms per square, as compared with 10⁸ ohms per square of a vinyl productof the Lindsay, et al. patent, and delamination is virtually impossible.Ambient humidity, and/or water immersion are found to have nosignificant effect on electrical properties. The inherent capability ofCondulon to discharge a 5 kV charge in 0.02 second enables designmodification to increase the speed of electrostatic discharge, from the0.05 second time observed for the described mat, either by creating aless-thick top layer 10 or by increasing the proportion of conductiveplasticizer in the low-conductivity layers 10-12. Generally, thethickness range of layer 10 may be between 25 and 50 mils, the thicknessrange of layer 11 may be between 1 and 5 mils, and the thickness rangeof layer 12 may be between 25 and 125 mils, expansion being optional anddependent upon ultimate use.

While the invention has been described in detail for the preferredembodiment, it will be understood that modifications may be made withoutdeparting from the scope of the invention.

What is claimed is:
 1. A three-ply conductive web for dissipating astatic electric charge impressed thereon, comprising an upper layer ofthermoplastic polymeric material having a volume resistivity of 1×10⁷ to1×10¹² ohm-cm, a thin conductive film of thermoplastic polymericmaterial laminated directly to said upper layer and having a surfaceresistance in the order of 10² ohms per square, and a conductiveunderlying lower layer of expanded thermoplastic polymeric material indirect electrical contact with said conductive film and having a volumeresistivity of 1×10⁷ to 1×10¹² ohm-cm.
 2. In a multi-layer conductiveweb for discharging a static electric charge impressed thereon, whereinan upper layer of thermoplastic polymeric material having a volumeresistivity of 1×10⁷ to 1×10¹² ohm-cm is in direct physical andelectrical connection with a more conductive underlying layer, andwherein conductive means in electrical contact with said underlyinglayer is provided to connect said underlying layer to ground potential,the improvement wherein said underlying layer is a continuous thinconductive film of thermoplastic polymeric material having a surfaceresistance in the order of 10² ohms per square, whereby the surfaceresistivity of the web as measured on the upper surface of said upperlayer is in the order of 10⁷ ohms per square, said conductive means inelectrical contact with said underlying layer being a layer of expandedthermoplastic polymer having a volume resistivity of 1×10⁷ to 1×10¹²ohm-cm.
 3. The improvement of claim 2, in which the thermoplasticpolymeric materials of each said upper layer and said underlying layercomprises essentially solid plasticized polyvinyl chloride with aconductive ingredient compounded therein, the conductive ingredient ofsaid upper layer being a salt and the conductive ingredient of saidunderlying layer being carbon black.
 4. The improvement of claim 3, inwhich the thickness of said underlying layer is in the range of 1 to 5mils, and the thickness of said upper layer is in the range of 25 to 50mils.
 5. The improvement of claim 4, in which the thickness of saidunderlying layer is substantially 2 mils.
 6. The improvement of claim 5,in which said underlying layer has a surface resistance of substantially300 ohms per square.
 7. In a multi-layer conductive web for discharginga static electric charge impressed thereon, wherein an upper layer ofthermoplastic polymeric material having a volume resistivity of 1×10⁷ to1×10¹² ohm-cm is in direct physical and electrical connection with amore conductive underlying layer, and wherein conductive means inelectrical contact with said underlying layer is provided to connectsaid underlying layer to ground potential, the improvement wherein saidunderlying layer is a continuous thin conductive film of thermoplasticpolymeric material having a surface resistance in the order of 10² ohmsper square, whereby the surface resistivity of the web as measured onthe upper surface of said upper layer is in the order of 10⁷ ohms persquare, said conductive means comprises a bottom layer of expandedplasticized polyvinyl chloride with a conductive compound therein andhaving a volume resistivity of 1×10⁷ to 1×10¹² ohm-cm.
 8. As an articleof manufacture, a multi-layer static-dissipating floor mat, comprisingan upper layer level of solid electrically conductive plasticizedpolyvinyl chloride having a volume resistivity of 10⁷ to 10¹² ohm-cm, anintermediate layer of thin continuous solid electrically conductiveplasticized polyvinyl chloride having a surface resistance in the orderof 10² ohms per square, and a bottom layer of expanded electricallyconductive plasticized polyvinyl chloride having a volume resistivity of10⁷ to 10¹² ohm-cm.